The objective of the research proposed in this supplemental application is to determine the chemical effect which vitamin K exerts on the blood-clotting glycoproteins (especially prothrombin). Vitamin K is required for the completion of the calcium-binding sites on prothrombin. Since calBium-binding is crucial to blood-coagulation, the chemical characterization of the calcium-binding groups will not only lead to further knowledge of the blood-clotting process and its control, but will also greatly advance our knowledge of the mode of action of vitamin K. It has been demonstrated that the calcium-binding region of prothrombin can be isolated as a relatively small peptide. The objectives of this work are to characterize this peptide and its attached prosthetic group and further to try to characterize the enzymology of the vitamin K-dependent reaction involving this region of prothrombin.